Lehrs for annealing and tempering of glassware are well known. The ware is carried through the lehr which is in the form of a tunnel by means of a conveyor belt. The glass is heated until it reaches annealing temperature, taking a few minutes to reach strain development; afterwards the glass is slowly cooled by temperature control until just below the strain point, below which the permanent strains acquired in cooling no longer exist. The ware is then cooled at a faster rate to ambient temperature.
Early lehrs were manufactured in an integrated manner, formed by huge sections which caused great problems in installation, maintenance and moving, if it were required. Glassware such as bottles, jars, etc., were passed through the lehr close to the tunnel entrance and slowly moved through the heating zone being afterwards passed to a tempering zone where the temperature was raised to around 560.degree. C. and later the articles were cooled until they reached handling temperature. Those lehrs had problems due to sudden entrance of air currents from the cooling zone to the tempering zone which caused either breakage of the articles or ineffective tempering, especially along the length of the cooling zone.
Subsequently the manufacture of lehrs began, using smaller, more manageable modules or sections with integrated heating and cooling systems, thereby gaining great advantages, in that it was no longer necessary to construct the lehr at its intended place of use because of the modular construction. Very efficient transversal cooling systems were created with uniform thermal treatment along the width of the lehr. Characteristically there were air movements inside the lehr impeding air currents along the lehr's length which would otherwise adversely affect the thermal treatment curves.
The developments that have thus far occured in lehrs have been oriented towards better thermal efficiency, faster conveyor belt speed, shorter process time, greater process control, and have tended toward higher productivity for these machines.
One example of prior art lehrs, may be found in U.S. Pat. No. 3,145,092 dated Aug. 18, 1964. This patent describes an improvement in lehrs, especially in the cooling of the glassware after heat has been applied. Basically, the cooling system is in independently regulated sections or zones, to provide constant cooling. The sections have a series of ducts forming right angles that are connected (one to each section) at one end to a transverse wall of the lehr and on the other end to a central duct which extends longitudinally along the upper part of the lehr in a section of the cooling zone. This central duct is connected to a vacuum fan at the entrance to maintain suction inside the cooling section. Each cooling section into which the lehr is divided is independently regulated to provide constant cooling of articles. The air directed inside the tunnel is supplied by a forced air fan, located at the end of the cooling zone.
Another U.S. Pat. No. 3,884,667 dated May 20, 1975, describes a lehr which cools articles by controlling temperature, as the articles pass longitudinally through the lehr. The lehr is formed of a plurality of sections and the articles pass through the various sections in a longitudinal direction by means of a conveyor belt. Each individual section consists of a peripheral insulation inside a rigid structure. Below the roof of each section is a fan which is connected by a shaft extending through the insulation panel to an electric motor. Over one or both sides of the section, heating or cooling elements are provided and the fan sucks the hot or cold air through the upper surface of the conveyor belt and the glassware placed on the belt, directing the air laterally towards the heating element. After passing through the heating element, the air current goes down and flows along the length of the external walls and again passes through the conveyor belt to continue circulating among the articles.
Still another U.S. Pat. No. 3,997,317 dated Dec. 14, 1976, describes a lehr constructed in modules or sections and arranged in a series to form a tunnel in which a conveyor belt is disposed for transporting glassware, such as bottles or other articles to be tempered. Each section has circulation and recirculation ducts above the conveyor belt connected to vertical ducts on opposite sides, with gas burners and electric heating elements, separately or closely fitted next to the vertical ducts. It also has fans positioned in each vertical duct which directs air downwardly toward the gas burners and/or electric heating elements, thereby heating the air, passing it to an area underneath the conveyor belt, and then directing it upwards to heat the articles. This patent also describes means to increase controlled quantities of ambient air at the inlet of each fan.
As can be seen from the lehrs described in the above patents, various types have been developed, each one directed toward increasing production and improving quality of tempering and/or annealing. The majority have heaters and fans in each section, and this increases the expense of energy, equipment, operating and maintenance. Such is the case of the Schraven patent, which uses, one or both sides of each section, heating elements and a fan; or like the Dicks patent, which uses gas burners and electric heating elements on opposite sides and fans positioned in each side.